Magnetic resonance imaging (MRI) is now coming into wide-spread commercial usage. Nevertheless, there are still many possible areas for improvement. For example, desired improvements are still sought to improve the signal-to-noise ratio in NMR responses and, accordingly, in resulting NMR images.
One previously known technique for improving the attainable signal-to-noise ratio involves the use of quadrature RF transmit/receive coils. For example, a general description of such quadrature RF coils and of the potential benefits to be derived from use of same is provided in the following prior publications:
"Quadrature Detection Coils--A Further Improvement in Sensitivity" by Chen et al, J. Mag. Res. 54, 324-327 (1983) PA0 "Quadrature Detection in the Laboratory Frame" by Hoult et al, Mag. Res. Med. 1 339-353 (1984) PA0 "A Quadrature Probe For Adult Head NMR Imaging" by Sank et al, Department of Radiology, NIH, pp 650-651 PA0 "Radio Frequency Penetration Effects in MR Imaging: Simulation/Experiment with Linearly Polarized and Circularly Polarized RF Fields" by Glover et al, GE Medical Systems, pp 264-265.
These prior publications describe a pair of matched quadrature RF coils wherein each coil includes four axially extending legs disposed at 60.degree., 120.degree., 60.degree. and 120.degree. spacing about a common cylinder--with one coil being rotated by 90.degree. spatially with respect to the other. The phase quadrature outputs from the two separate coils are subsequently combined in a 90.degree. hybrid so as to produce an output having increased signal-to-noise ratio (e.g., because the non-coherent noise will tend to cancel when the two signals are coherently added with an appropriate 90.degree. phase shift in one of them). As is also noted in these references, one or both of the coils may be advantageously utilized for transmitting RF NMR excitation pulses into the enclosed volume to be imaged so as to further enhance the attainable signal-to-noise ratio and/or so as to reduce the required level of transmitted RF power.
Ideally, there should be no inductive coupling between the two RF quadrature coils. In reality, there is always inherently some spurious coupling. Nevertheless, the effective isolation between the two coils can be improved by purposefully adding some additional coupling between the coils of the proper amplitude and phase to cancel (or at least substantially reduce) the unwanted but inherent spurious inter-coil coupling. It is apparently for this purpose that the prior art has employed conductive areas (termed "paddles") between some sections of the coil legs.
Although the general theory of quadrature detection coils is known in the prior art, the successful realization of a commercially reproducible working embodiment of such a system with minimum coupling between the coils, an RF balanced coil structure--and one which is nevertheless configured spatially in a manner which facilitates not only manufacture but actual use--remains as a difficult task.
Some prior approaches have employed coil structures having both ends of the coil structure open-ended and with axially extending feedlines emanating from opposite ends of the overall structure and respectively associated with each of the two coils.